Abstract
The alkyl group migration between the acyl carbon atom and a sulfur donor atom of a series of rhodium carbonyl dithiolates has been explored by synthesis, spectroscopic characterization, and X-ray structure determination of the typical S-alkylated complex Rh(CO)(PPh 3 )(Et-mnt) (mnt = maleonitriledithiolate; Et-mnt = the S-ethylated derivative). The neutral rhodium(III) acyl complexes Rh(COR)LL′(mnt) where L = PPh 3 and PEt 3 and L′ = pyr, C 6 H 11 NH 2 , and Sol are prepared by the reaction of [Rh(CO)L(mnt)] - with RX followed by removal of X - with Ag + and the addition of L′. Heating of these neutral acyl complexes in THF or CHCl 3 solution leads in certain cases to the S-alkylated derivatives Rh(CO)(PPh 3 )(R-mnt) (R = Et, n-Pr, Bz) and the corresponding PEt 3 complex where R = Et. The migration was followed in both the forward and reverse directions by IR and NMR spectroscopies, and in one case where L = PPh 3 , L′ = pyr, and R = Et an equilibrium mixture of the two Rh species was obtained. The complex Rh(CO)(PPh 3 )(Et-mnt) was examined by single-crystal X-ray methods and refined by standard procedures to final agreement factors R and R′ of 3.73 and 5.14%. The geometry of the complex is square planar with the alkylated sulfur donor cis to the carbonyl ligand. The alkylated S donor is pyramidal, and the bond distances and angles are as expected. The S-alkylated complex is a Rh(I) system, and hence the alkyl group migration takes place with an internal redox reaction in which an electron pair of the dithiolate ligand serves in a complementary way to the electron pair of the Rh(III)/Rh(I) couple. The mechanism of the alkyl group migration is proposed as a succession of two 1,2 shifts with a rhodium(III) alkyl carbonyl dithiolate as an intermediate. © 1979 American Chemical Society.